This invention relates to a semiconductor laser element comprising a semiconductor laser crystal and a heat sink.
A conventional semiconductor laser element comprises a semiconductor laser crystal attached to a heat sink. The laser element is further attached to a larger heat sink to form a semiconductor laser. When used as a semiconductor laser crystal, a GaAs-Al.sub.x Ga.sub.1-x As double heterojunction crystal is featured by its capability of producing a continuous optical output at room temperature. This material is being put into use in lasers since the crystal defects that once were found in these crystals and, which resulted in the deterioration of laser performance, have been largely eliminated. The exposed reflection surfaces (Fabry-Perot reflection surfaces) of the laser crystal, however, undergo deterioration when the semiconductor laser is continously kept in operation for more than 1,000 hours and deterioration of laser operation has been found to result from a gradual erosion and degeneration during the operation of an active layer which has ends exposed on the reflection surfaces. The damage caused to the exposed active layer ends is serious when the laser is put into operation in a more humid and oxidizing atmosphere, which atmosphere is unavoidable in many practical applications of semiconductor lasers.
It has therefore been proposed to protect the active layer ends against the atmosphere by the provision of a thin covering film as described, for example, in an article contributed by H. Kressel et al in "RCA Review," Volume 36, pages 230-239 (June, 1975), under the title of "Reliability Aspects and Facet Damage in High-Power Emission from (AlGa)As CW Laser Diodes at Room Temperature." A similar protective film is described by Akihiro Tomozawa et al for a plurality of transistors, an IC, and an LSI in U.S. Pat. No. 3,935,083. No effective covering film, however, has yet been realized for a semiconductor laser crystal.
An attempt has also been made to adjust the reflectivity of each reflection surface by covering the reflection surface with a dielectric film of a suitable thickness. The dielectric material may be silicon monoxide, silicon dioxide, or the like. Yet another attempt made has been to evaporate a metal film on the dielectric film to achieve perfect reflection. It is also known that a dielectric film formed on each reflection surface of a semiconductor laser crystal to reduce the reflectivity rather than raise the same is effective in avoiding the so-called optical or mirror-surface damage, which is an instantaneous damage caused, on making the laser crystal produce a large optical output, to the reflection surfaces by the intensity of the produced light besides the gradual deterioration of the exposed active layer ends. The reduction in the reflectivity renders the production of the optical output easier (reduces the light intensity within the laser crystal) and thereby avoids the optical damage.
Each reflection surface of a semiconductor laser crystal is about 100.times.100 to 200 microns wide and the active layer end exposed in each reflection surface is about 5.times.100 to 200 microns wide. The dielectric film should be sufficiently wide to cover each exposed active layer end. In practice, the dielectric film is generally formed on the whole reflection surface because of the difficulty of forming the dielectric film only on the limited areas of the exposed active layer ends. Even if it were formed on the entire reflection surface, the dielectric film is still insufficient to protect the reflection surfaces against deterioration or damage. A semiconductor laser for producing a continuous optical output at room temperature is subjected to very severe conditions of operation and is mainly referred to hereunder as an example of a semiconductor laser because the protection of the exposed reflection surfaces of a semiconductor laser crystal of such a laser well applies to the protection of other semiconductor laser crystals.
A conventional semiconductor laser crystal for producing a continuous optical output at room temperature is attached to a heat sink of, for example, diamond of the IIa type or copper by an interposed tin or indium layer. The tin plating shown in U.S. Pat. No. 3,733,561 issued to Izuo Hayashi on a diamond heat sink appears to show such an interposed layer. the attaching step is carried out at a temperature of about 250.degree. C. for tin or of about 200.degree. C. for indium so as not to undesirably introduce strains into the laser crystal. The above-mentioned dielectric film is attached to the laser crystal surfaces prior to the attachment of the laser crystal to the heat sink. Inasmuch as the dielectric film does not sufficiently tenaciously adhere to the crystal surfaces by nature and has a coefficient of thermal expansion that is different from that of the laser crystal, either the film tends to exfoliate from the crystal surfaces or the strength of adherence of the film to the crystal surfaces is considerably weakened during the subjection of the semiconductor laser crystal to a heat cycle of in the laser crystal attaching step. Even when the laser crystal is placed on the heat sink, the dielectric film is often damaged by the pincette, which is used in this operation, or is otherwise contaminated. Furthermore, it is very difficult and barely possible, even with an objectionably low yield to form a dielectric film on such a small area as exemplified hereinabove. These disadvantages of the conventional dielectric film are also present in the thin covering film formed on each reflection surface to protect the exposed active layer end from the atmosphere. in addition, an interposed tin or indium layer is oxidized in a humid and oxidizing atmosphere to reduce its strength of attaching the semiconductor laser crystal to the heat sink, thereby to shorten the operation and storage life of the semiconductor laser.